Known examples of a friction welding apparatus of this type are disclosed in Patent Documents 1 and 2. Patent Document 1 discloses a technique in which a stopping device is operated through clutch engagement to perform phase adjustment on both workpieces. Patent Document 2 discloses a technique in which phase adjustment is performed on both workpieces through torque control and rotation frequency control of a servo motor.    Patent Document 1: JP 3-184680 A    Patent Document 2: JP 2000-84680 A
However, in Patent Document 1, the rotation frequency at which the engagement of both clutches is effected is, for example, 300 rpm, so that when both clutches are engaged with each other, the engagement portions may suffer breakage. Further, this engagement involves generation of large impulsive noise. For example, when there are two engagement portions for both clutches with respect to one rotation of the spindle side clutch, that is, when there exists an engagement portion for both clutches for each half rotation)(180° of the spindle side clutch, the engagement is effected without detecting the phases of both clutches, so that when both clutches are brought into engagement with each other, the spindle side clutch may rotate by 1° with respect to the mating clutch before being engaged after both clutches have started to be brought into contact with each other, or the spindle side clutch may rotate by 179° before being engaged. In the former case (i.e., when the spindle side clutch rotates by 1° before being engaged), the loss of rotational energy of the spindle due to the clutch engagement is small, so that it is possible to cause the stopping device to perform a predetermined operation with the rotational energy of the spindle. In this case, the phase precision in the bonding of both workpieces is satisfactory. On the other hand, in the latter case (i.e., when the spindle side clutch rotates by 179° before being engaged), the rotational energy loss of the spindle due to the clutch engagement is large, so that it is impossible to cause the stopping device to perform the predetermined operation with the rotational energy of the spindle. In this case, the phase precision in the bonding of both workpieces is unsatisfactory. In this way, the phase precision in the bonding of both workpieces may be satisfactory or unsatisfactory, resulting in variation in phase precision.
In Patent Document 2, immediately before the bonding of both workpieces, the electricity supply to the servo motor for rotating the spindle is cut off (i.e., servo-OFF is effected), thereby stopping the spindle. As a result, from the servo-OFF onward, the spindle can be rotated by inertial force, so that both workpieces are allowed to adhere to each other depending on the softened state (i.e., depending on the viscosity) of the contact portions thereof. That is, there is a new problem in that both workpieces are bonded together before a point has been reached where the phases of both workpieces coincide with each other, or after a point has been reached where the phases of both workpieces coincide with each other.
Thus, there is a need in the art for a friction welding apparatus which helps to achieve an improvement in terms of the bonding phase precision of both workpieces.